How do I turn off the automatic update of the date/time clock

I am running a home network, using 2 Dell computers. My internet connection is DSL through a Siemens Gigaset SE 567 modem. I have set the windows date/time clock to use the internet time up date option, which should happen once every seven days. I have also turned the time client function of the modem off. The clock still changes the date/time clock every 3 minutes. This causes a security program, which uses the date/time clock to make file headers,to crash